U.S. patent number 7,769,327 [Application Number 12/165,852] was granted by the patent office on 2010-08-03 for developing device and image forming apparatus having a developing roller with a grooved sleeve.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Kazuma Hinoue, Kiyofumi Morimoto, Hiroo Naoi, Kohichi Takenouchi, Mitsuru Tokuyama.
United States Patent |
7,769,327 |
Morimoto , et al. |
August 3, 2010 |
Developing device and image forming apparatus having a developing
roller with a grooved sleeve
Abstract
A developing device includes a developing roller and a blade.
The developing roller includes: a developing sleeve which rotates
while holding two-component developer composed of toner and
magnetic carrier on the outer surface having a plurality of grooves
extending in parallel to an axis of the rotation; and a magnetic
member, provided inside the developing sleeve in such a manner as
to be unrotatable, for attracting the two-component developer onto
the outer surface of the developing sleeve. The blade is provided
outside the rotating sleeve with a gap from the outer surface of
the developing sleeve for scraping off a part of the two-component
developer on the outer surface of the developing sleeve. Also, the
magnetic member includes a magnet for controlling magnetic flux
density in the gap to range from 70 mT to 150 mT. This allows
preventing image degradation caused by generation of development
memory.
Inventors: |
Morimoto; Kiyofumi (Tenri,
JP), Takenouchi; Kohichi (Tenri, JP),
Tokuyama; Mitsuru (Kizugawa, JP), Naoi; Hiroo
(Nara, JP), Hinoue; Kazuma (Yamatokoriyama,
JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
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Family
ID: |
40213484 |
Appl.
No.: |
12/165,852 |
Filed: |
July 1, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090010683 A1 |
Jan 8, 2009 |
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Foreign Application Priority Data
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Jul 3, 2007 [JP] |
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2007-175315 |
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Current U.S.
Class: |
399/276;
430/111.3; 399/267; 399/277 |
Current CPC
Class: |
G03G
15/0812 (20130101); G03G 15/0921 (20130101); G03G
15/0928 (20130101) |
Current International
Class: |
G03G
15/09 (20060101) |
Field of
Search: |
;399/274,275,276,277,267
;430/110.4,111.3,111.32,122.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2004-170555 |
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Jun 2004 |
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JP |
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2005-24682 |
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Jan 2005 |
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JP |
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2006-65317 |
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Mar 2006 |
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JP |
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2006-099029 |
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Apr 2006 |
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JP |
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2006-099029 |
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Apr 2006 |
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JP |
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2006-184475 |
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Jul 2006 |
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JP |
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2007-025463 |
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Feb 2007 |
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JP |
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2007-079117 |
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Mar 2007 |
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JP |
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Primary Examiner: Chen; Sophia S
Attorney, Agent or Firm: Nixon and Vanderhye, PC
Claims
What is claimed is:
1. A developing device comprising: a developing roller, including:
a rotating sleeve which rotates while holding two-component
developer composed of toner and magnetic carrier on an outer
surface having a plurality of grooves that extend in a direction
parallel to an axis of the rotation, wherein a pitch between
adjacent grooves on the outer surface of the rotating sleeve ranges
from 1.25 mm to 4 mm, wherein a width of each groove ranges from
0.2 mm to 0.35 mm, and wherein a depth of each of the grooves
ranges from 0.08 mm to 5.00 mm, and a magnetic member, provided
inside the rotating sleeve in such a manner as to be unrotatable,
for attracting the two-component developer onto the outer surface
of the rotating sleeve; and a blade, provided outside the rotating
sleeve with a gap from the outer surface of the rotating sleeve,
for scraping off a part of the two-component developer, the
magnetic member including a magnet for controlling a magnetic flux
density in the gap to range from 85 mT to 150 mT.
2. The developing device as set forth in claim 1, wherein: each of
the grooves has a rectangular shape in a cross section of the
rotating sleeve when the rotating sleeve is cut by a plane
perpendicular to the axis.
3. The developing device as set forth in claim 1, wherein: the
magnet is provided between the axis and the blade.
4. The developing device as set forth in claim 3, wherein: the
blade is made of a magnetic material.
5. The developing device as set forth in claim 1, wherein: a volume
average particle size of the toner ranges from 4 .mu.m to 7
.mu.m.
6. The developing device as set forth in claim 1, wherein: a volume
average particle size of the magnetic carrier ranges from 20 .mu.m
to 60 .mu.m.
7. The developing device as set forth in claim 1, wherein: the
magnetic carrier is obtained by coating ferrite-based core
particles with thermosetting resin.
8. The developing device as set forth in claim 1, wherein:
saturated magnetization of the magnetic carrier ranges from 30
emu/g to 70 emu/g.
9. The developing device as set forth in claim 1, wherein the
magnetic flux density in the gap ranges from 100 mT to 150 mT.
10. An image forming apparatus comprising a developing device
including: a developing roller that includes: a rotating sleeve
which rotates while holding two-component developer composed of
toner and magnetic carrier on an outer surface having a plurality
of grooves that extend in a direction parallel to an axis of the
rotation, wherein a pitch between adjacent grooves on the outer
surface of the rotating sleeve ranges from 1.25 mm to 4 mm, wherein
a width of each groove ranges from 0.2 mm to 0.35 mm, and wherein a
depth of each of the grooves ranges from 0.08 mm to 5.00 mm, and a
magnetic member, provided inside the rotating sleeve in such a
manner as to be unrotatable, for attracting the two-component
developer onto the outer surface of the rotating sleeve; and a
blade, provided outside the rotating sleeve with a gap from the
outer surface of the rotating sleeve, for scraping off a part of
the two-component developer, the magnetic member including a magnet
for controlling the magnetic flux density in the gap to range from
85 mT to 150 mT.
11. The image forming apparatus as set forth in claim 10, wherein
the magnetic flux density in the gap ranges from 100 mT to 150 mT.
Description
This Nonprovisional application claims priority under 35 U.S.C.
.sctn.119(a) on Patent Application No. 175315/2007 filed in Japan
on Jul. 3, 2007, the entire contents of which are hereby
incorporated by reference.
FIELD OF THE TECHNOLOGY
The present technology relates to a developing device included in
an electrophotographic image forming apparatus.
BACKGROUND OF THE TECHNOLOGY
In electrophotographic image forming apparatuses such as a
multifunction printer, a copying machine, a printer, and a
facsimile, a developing process is carried out by providing
developer to a latent image formed on a surface of a photoreceptor.
Various methods are available as a developing process. Among them,
a magnetic brush developing method by using two-component developer
composed of toner and magnetic carrier has been widely used because
it is excellent in high-speed capability. Note that two-component
developer is merely called developer and magnetic carrier is called
carrier hereinafter.
A developing device in a magnetic brush developing method includes
a developer tank and a magnetic developing roller, as illustrated
in FIG. 1 in the following Patent Citation 1. The developing roller
magnetically attracts (scoops) developer stored inside the
developer tank, and holds the developer on an external surface of
the developing roller, and then conveys the developer to a
photoreceptor. This procedure achieves the developing process.
Recently, minimization of the particle size in toner and carrier,
both of which are contained in developer, is positively carried out
to improve image quality. However, as particle sizes of toner and
carrier are minimized, flowability of developer drops, resulting in
a problem such as decline in image density because the developing
roller fails to attract enough developer from the developer tank.
An example of a technique to solve this problem is a developing
device in the Patent Citation 1. In the Patent Citation 1, a
developing device includes a developing roller composed of: a
developing sleeve (axis sleeve) with a plurality of grooves on an
outer surface that extend in a direction of a rotation axis; and a
magnet roller inserted in a hollow inside the developing sleeve,
and the distance between the grooves and the depth of each groove
are set to be within predetermined ranges, in order to improve the
attracting effect (scooping effect) of developer by the developing
roller.
[Patent Citation 1] Japanese Unexamined Patent Publication, Tokukai
2004-170555 (date of publication: Jun. 17, 2004)
However, in a developing device, which includes a developing sleeve
having a plurality of grooves on the outer surface, the following
adverse effect was found. After printing an image partially having
an area with extremely high density, another image printed at a
given density partially has an area with lower density than the
given density.
The following explains this adverse effect. In a magnetic brush
developing method, normally, a bias is applied to a developing
roller during a developing process. This allows a potential of
developer on the external surface of the developing sleeve to be
substantially uniform.
When there is performed first printing in which an image partially
having an area with extremely high density (central part in the
main scanning direction in FIG. 1 (a)) is printed as shown by
reference number 300 in FIG. 1(a) and then the developing device is
driven, an electric potential difference of developer is generated
on the outer surface of the developing sleeve 200 between (i) area
".alpha." which is a central part in the main scanning direction
(corresponding to the area with extremely high density in the first
printing) and (ii) area "b" which is a part other than the area
".alpha.", as illustrated in FIG. 1(b). Specifically, an electric
potential of developer on the area ".alpha." corresponding to the
area with extremely high density in the first printing is lower
than that of the developer on the area "b". Note that such
phenomenon of potential differences is called "development memory"
in this specification.
Assume that after the development memory illustrated in FIG. 1(b)
is generated on the developing sleeve 200 by carrying out the first
printing, for example, there is performed second printing in which
a solid image having substantially uniform density. In this case,
the potential of developer on the area ".alpha." is lower than the
potential of developer on the area "b". Therefore, in a
photosensitive drum, a potential of a part having toner provided
from the area ".alpha." is lower than that of a part having toner
provided from the area "b". As a result, in an image transferred on
a sheet, an area printed with the toner provided from the area
".alpha." has lower density than the area printed with the toner
provided from the area "b". Namely, in the case of printing a solid
image having substantially uniform density by the developing sleeve
200 in which the development memory is generated as illustrated in
FIG. 1 (b), a central part in the main scanning direction has lower
density than areas on both sides of the central part, as shown by a
reference number 400 in FIG. 1 (a).
As explained above, in the case where an image partially having an
area with extremely high density is printed, the development memory
is generated in the developing sleeve. Thereafter, when an image is
printed at a given density, the image partially has an area with
lower density than the given density due to the development memory.
Therefore, it is apparent that unless the generation of the
development memory is prevented, density unevenness is generated,
resulting in the degradation of the printed image.
SUMMARY OF THE TECHNOLOGY
The objective of the present technology is to prevent the
degradation of a printed image caused by the generation of the
development memory in a developing device including a developing
roller with a rotating sleeve.
The inventors have studied how to prevent the development memory
generated in a rotating sleeve included in a developing device. As
a result of diligent studies, the inventors found out that the
generation of the development memory at the developing sleeve can
be prevented by the developing device including: a developing
roller including a rotating sleeve which rotates while holding
two-component developer composed of toner and magnetic carrier on
an outer surface having a plurality of grooves that extend in a
direction parallel to an axis of the rotation, and a magnetic
member, provided inside the rotating sleeve in such a manner as to
be unrotatable, for attracting the two-component developer onto the
outer surface of the rotating sleeve; and a blade provided outside
the rotating sleeve with a gap from the outer surface of the
rotating sleeve for scraping off a part of the two-component
developer, the magnetic member including a magnet for controlling a
magnetic flux density in the gap to range from 70 mT to 150 mT.
Accordingly, this developing device has an effect of suppressing
degradation in a printed image caused by the generation of the
development memory.
Additional objectives, features, and strengths of the technology
will be made clear by the description below. Further, the
advantages will be evident from the following explanation in
reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 (a) is a drawing schematically illustrating an image
test-printed by an image forming apparatus.
FIG. 1 (b) is a drawing illustrating a developing sleeve in which
the development memory is generated
FIG. 2 is a drawing illustrating an internal structure of an image
forming apparatus in accordance with an embodiment of this
technology.
FIG. 3 is a drawing illustrating a detailed structure of the black
image forming unit illustrated in FIG. 2.
FIG. 4 is a drawing illustrating a detailed structure of the
developing device illustrated in FIG. 3.
FIG. 5 is a detailed cross-section view illustrating the developing
sleeve illustrated in FIG. 4.
FIG. 6 is an explanatory drawing illustrating a mechanism in which
the amount of developer on an outer surface of the developing
sleeve is controlled by a doctor blade.
FIG. 7 (a) is an explanatory drawing illustrating a toner film
formed on an entire outer surface of the developing sleeve.
FIG. 7 (b) is an explanatory drawing illustrating a magnetic brush
adhered onto the toner film formed on the entire outer surface of
the developing sleeve.
FIG. 7 (c) is an explanatory drawing illustrating that a part of
the toner film is detached from the outer surface of the developing
sleeve.
FIG. 7 (d) is an explanatory drawing illustrating that magnetic
brush adheres onto the toner film and the outer surface of the
developing sleeve in a case where a part of the toner film is
detached from the outer surface of the developing sleeve.
FIG. 8 (a) is an explanatory drawing illustrating a part of the
developer adhered onto the outer surface of the developing sleeve
is scraped off by a doctor blade in the developing device in
accordance with an embodiment of this technology.
FIG. 8 (b) is an explanatory drawing illustrating that a part of
the developer adhered onto the outer surface of the developing
sleeve is scraped off by a doctor blade in a conventional
developing device.
DESCRIPTION OF THE EMBODIMENTS
Structure of Image Forming Apparatus
The following explains an embodiment of this technology with
reference to FIG. 2 through FIG. 8. FIG. 2 is a schematic view
illustrating an internal structure of an image forming apparatus.
As illustrated in FIG. 2, an image forming apparatus 50 is a tandem
engine color printer including a black image forming unit 1 for
forming a black toner image, a cyan image forming unit 2 for
forming a cyan toner image, a magenta image forming unit 3 for
forming a magenta toner image, a yellow image forming unit 4 for
forming a yellow toner image.
Above these four image forming units 1 through 4, an intermediate
transfer belt (endless belt) 5 is provided. The intermediate
transfer belt 5 is suspended by two supporting rollers 6 and
rotates in a direction indicated by an arrow R. As a material of
the intermediate transfer belt 5, resins such as polyimide,
polyamide, and the like, mixed with an appropriate amount of a
conductive material may be used.
Outside the intermediate transfer belt 5, the black image forming
unit 1, the cyan image forming unit 2, the magenta image forming
unit 3, and the yellow image forming unit 4 are arranged in this
order from the upstream to the downstream of the rotation direction
R.
Inside the intermediate transfer belt 5, a plurality of first
transfer rollers 7 for transferring single-color toner images
formed in the image forming units 1 through 4 to the intermediate
transfer belt 5 are provided so as to face the image forming units
1 through 4, respectively. Single-color toner images formed in the
image forming units 1 through 4 are transferred to the intermediate
transfer belt 5 so that the single-color toner images overlap one
another. This allows forming a multi-color image.
Also, outside the intermediate transfer belt 5, a second transfer
roller 8 for transferring the multi-color image formed on the
intermediate transfer belt 5 to a sheet of paper (paper medium) is
provided at downstream of the yellow image forming unit 4 with
regard to the rotation direction R.
Further, outside the intermediate transfer belt 5, a belt cleaning
unit 10 for cleaning the surface of the intermediate transfer belt
5 is provided at downstream of the second transfer roller 8 with
regard to the rotation direction R. The belt cleaning unit 10
includes: a belt cleaning brush 11, which is provided to contact
the intermediate transfer belt 5; and a belt cleaning blade 12,
which is located at downstream of the belt cleaning brush 11 with
regard to the rotation direction R.
Further, below the image forming units 1 through 4, a tray 14 for
storing paper is provided. A sheet of paper in the tray 14 is fed
by a plurality of feeding rollers 13 to a second transfer zone 30
in which the second transfer roller 8 and the intermediate transfer
belt 5 face each other. Note that the direction indicated by an
arrow P in FIG. 2 is a direction in which a sheet is fed.
Further, at the downstream of the second transfer roller 8 with
regard to the paper feeding direction P, a fixing unit 15 is
provided for fixing the transferred image onto a sheet of paper.
Also, at further downstream of the fixing unit 15 with regard to
the paper feeding direction P, ejection rollers 13a are provided
for outputting a sheet on which the image is fixed from the image
forming apparatus 50.
In the image forming apparatus 50, the image forming units 1
through 4 transfer single-color toner images, respectively, to the
intermediate transfer belt 5 so that a multi-color image is formed
on the intermediate transfer belt 5. Then, the multi-color image on
the intermediate transfer belt 5 is secondary transferred at the
second transfer zone 30 to a sheet of paper fed by the feeding
rollers 13, and then the transferred image is fixed onto the paper
at the fixing unit 15. Thereafter, the paper on which the
multi-color image is fixed is outputted from the image forming
apparatus 50 by the ejection rollers 13a. On the other hand,
residual toner remaining on the intermediate transfer belt 5 which
was not transferred to the paper, is cleaned by the belt cleaning
unit 10.
[Structure of the Image Forming Unit]
The following explains structures of the image forming units 1
through 4 in detail. FIG. 3 illustrates a detailed structure of the
black image forming unit 1 illustrated in FIG. 2. Note that the
cyan image forming unit 2, the magenta image forming unit 3, and
the yellow image forming unit 4 are substantially equal to the
black image forming unit 1 except for a toner color which each
image forming unit deals with. Therefore, the following explains
only a structure of the black image forming unit 1 and explanations
of the structures of the cyan image forming unit 2, the magenta
image forming unit 3, and the yellow image forming unit 4 are
omitted here.
As illustrated in FIG. 3, the black image forming unit 1 includes:
a photosensitive drum 16; a charging unit 17 at a periphery of the
photosensitive drum 16 for charging same; an exposure unit 18 for
writing an electrostatic latent image on an outer surface of the
photosensitive drum 16; a developing device 19 for visualizing
(developing) the electrostatic latent image written on the outer
surface of the photosensitive drum 16; and a photosensitive drum
cleaner 20 for cleaning residue such as residual toner remaining on
the outer surface of the photosensitive drum 16 after the first
transfer.
The photosensitive drum 16 includes a metal drum made of aluminum
and the like as a base, and a photoconductive layer made of organic
photoconductor (OPC), amorphous silicon (a-Si), and the like which
is a thin film formed on the outer surface of the metal drum.
The charging unit 17 is a scorotron charging unit which charges the
outer surface of the photosensitive drum 16 to have a certain
potential via corona discharge. Note that the charging unit 17 is
not limited to a scorotron charging unit and may be a contact
charging unit including a charging roller or a charging brush.
The exposure unit 18 is a laser scanning unit (LSU) which exposes
the outer surface of the photosensitive drum 16 by emitting laser
light in response to an image signal, and changes a surface
potential of the photosensitive drum 16 charged by the charger 17,
thereby forming an electrostatic latent image on the outer surface
of the photosensitive drum 16 in accordance with image information.
Note that the exposure unit 18 is not limited to a laser scanning
unit and may be a light emitting diode (LED) array.
The developing device 19 stores two-component developer (called
"developer" hereinafter) composed of toner and magnetic carrier
(called "carrier" hereinafter) and develops an electrostatic latent
image formed on the photosensitive drum 16 by using the toner
contained in the two-component developer. A structure of the
developing device 19 is detailed later.
The photosensitive drum cleaner 20 includes a cleaning blade 21, a
cleaner housing 22, and a seal 23.
The cleaning blade 21 is provided on the outer surface of the
photosensitive drum 16 in such a manner that the cleaning blade 21
and the outer surface of the photosensitive drum 16 press against
each other in order to clean residue remaining on the outer surface
of the photosensitive drum 16. As illustrated in FIG. 3, the
cleaning blade 21 is provided to press against the outer surface of
the photosensitive drum 16 in such a manner as to make an acute
angle between the cleaning blade 21 and the photosensitive drum 16
at the downstream of the cleaning blade 21 (i.e. at the downstream
of the rotation direction Rd of the photosensitive drum 16).
The cleaner housing 22 is for storing the residue scraped off by
the cleaning blade 21. Note that the cleaning blade 21 is mounted
in the cleaner housing 22.
The seal 23 is for sealing up inside the cleaner housing 22. One
end of the seal 23 is fixed with the cleaner housing 22 at the
upstream of the cleaning blade with regard to the rotation
direction Rd of the photosensitive drum 16, and the other end of
the seal 23 is in contact with the photosensitive drum 16.
[Structure of a Developing Device]
The following explains a structure of the developing device 19 in
detail. FIG. 4 is a schematic view of the developing device 19
illustrated in FIG. 3.
As illustrated in FIG. 4, the developing device 19 includes: a
developer tank 27 for storing developer; a developing roller 24 on
which a developing bias is applied and which supplies developer to
the photosensitive drum 16; a doctor blade (blade) 35 for
controlling thickness of a developer layer by scraping off extra
developer adhered onto the external surface of the developing
sleeve 26; and a stirring and carrying member 28 for stirring
developer inside the developer tank 27 and carrying the developer
to the developing roller 24.
The developing roller 24 is provided at an opening section 31 of
the developer tank 27 in such a manner as to face the
photosensitive drum 16 with a gap between the developing roller 24
and the photosensitive drum 16 (see FIG. 3), and attracts developer
stored inside the developer tank 27 to the outer surface thereof
and holds the attracted developer there. Thereafter, the developing
roller 24 rotates to carry and supply the attracted developer to
the outer surface of the photosensitive drum 16. Namely, the
developing roller 24 scoops up the developer inside the developer
tank 27 to the outer surface of the photosensitive drum 16.
The developing roller 24 includes a magnet roller (magnetic member)
25 with a cylindrical shape and a developing sleeve (rotating
sleeve) 26, which is provided to surround the outer surface of the
magnet roller 25 and to be rotatable in a direction of an arrow T
(counterclockwise direction).
The magnet roller 25 is a hollow and cylindrical magnetizing
member, which is provided inside the developing sleeve 26 in such a
manner as to be unrotatable by fixing ends of the magnet roller 25
in an axis direction to side walls of the developer tank 27,
respectively. Also, on an internal surface of the magnet roller 25,
a plurality of magnets are provided in the circumferential
direction with a distance between each other. The magnet roller 25
is magnetized by these magnets.
In a structure example illustrated in FIG. 4, the magnet roller 25
includes magnets N1, N2, and N3 which form the north pole of the
magnet roller 25, and magnets S1 and S2 which form the south pole
of the magnet roller 25.
The magnets N1, N2, N3, S1, and S2 are provided in such a manner
that one end of each magnet is in contact with the internal surface
of the magnet roller 25, and the other end of each magnet is away
from the internal surface of the magnet roller 25. The north pole
of the magnets N1, N2, and N3 are in contact with the internal
surface of the magnet roller 25, and the south pole of the magnets
S1 and S2 are in contact with the internal surface of the magnet
roller 25.
The magnet N1 is provided between the central axis of the
photosensitive drum 16 and the central axis of the magnet roller
25, and the magnetic flux density is set to be 110 mT (milli Tesla)
on peak.
The magnet N2 is provided at the position rotating by 117 degrees
from the position of the N1 magnet in a direction opposite to the
direction indicated by T, and the magnetic flux density is set to
be 56 mT on peak.
The magnet N3 is provided at the position rotating by 224 degrees
from the position of the N1 magnet in the direction opposite to the
direction indicated by T, and the magnetic flux density is set to
be 42 mT on peak.
The magnet S2 is provided at the position rotating by 282 degrees
from the position of the N1 magnet in the direction opposite to the
direction indicated by T, and the magnetic flux density is set to
be 80 mT on peak.
The magnet S1 is provided at the position that allows the magnetism
of the magnet S1 to affect a gap 36 between the outer surface of
the developing sleeve 26 and the edge of the doctor blade 35, and
the magnetic flux density is set to be 85 mT on peak. Specifically,
the magnet S1 is provided between the rotation axis of the
developing sleeve 26 and the edge of the doctor blade 35, at the
position rotating by 59 degrees from the position of the N1 magnet
in the direction opposite to the direction indicated by T.
Note that the peak value of the magnetic flux density of a magnet
means a magnetic flux density measured at the point closest to the
magnet on the outer surface of the developing sleeve 26. In the
developing device 19 of the present embodiment, the point closest
to the magnet S1 on the outer surface of the developing sleeve 26
is the gap 36. Therefore, the peak value of the magnetic flux
density at the gap 36 is 85 mT.
As illustrated in FIG. 4 and FIG. 5, the developing sleeve 26
includes a plurality of grooves on its outer surface which extend
parallel to the rotation axis direction of the developing sleeve
26, and is an axis blade made of non-magnetic material.
The doctor blade 35 is provided outside the developing sleeve 26 in
such a manner as to leave the gap 36 from the outer surface of the
developing sleeve 26. The gap 36 between the edge of the doctor
blade 35 and the outer surface of the developing sleeve 26 is set
to be 0.3 mm to 1 mm.
With the developing device 19 in FIG. 4 as described above,
developer stored inside the developer tank 27 is magnetically
attracted and is adhered onto the outer surface of the developing
sleeve 26 by the magnetism generated by the magnet N2 inside the
magnet roller 25. Thereafter, the developer adhered onto the outer
surface of the developing sleeve 26 is carried to the outer surface
of the photosensitive drum 16 through the gap 36 by the rotation of
the developing sleeve 26.
When passing through the gap 36, a part of the developer adhered
onto the outer surface of the developing sleeve 26 is scraped off
by the doctor blade 35 in combination with the magnetic force
generated by the magnet S1. This allows thickness of a developer
layer adhered onto the outer surface of the developing sleeve 26 to
be controlled.
The following explains in detail the mechanism how developer is
scraped off by the doctor blade 35 in combination with magnetic
force generated by the magnet S1 with reference to FIG. 6. FIG. 6
is an explanatory drawing illustrating the mechanism how the amount
of developer adhered onto the outer surface of the developing
sleeve 26 is controlled by the doctor blade 35. As illustrated in
FIG. 6, in the vicinity of the gap 36 between the outer surface of
the developing sleeve 26 and the edge of the doctor blade 35, a
magnetic brush 40 made of carrier is formed under the influence of
the magnetism of the magnet S1. The magnetic brush 40 is formed in
such a manner as to draw lines of magnetic force of the magnet S1
like standing spikes and is magnetically attracted and adhered onto
the outer surface of the developing sleeve 26. Also, toner is
adhered around the magnetic brush 40.
Thereafter, the magnetic brush 40, which is adhered on the outer
surface of the developing sleeve 26, is collided with the doctor
blade 35 while going through the gap 36 by the rotation of the
developing sleeve 26 (rotation in the direction T), and is broken
by the collision impact. As a result, the amount of developer
adhered onto the outer surface of the developing sleeve 26 is
controlled because a part of the magnetic brush 40 is scraped off
by the doctor blade 35 and the amount of developer per unit area is
reduced at the downstream of the gap 36 with regard to the
direction T compared with the amount of developer per unit area at
the upstream of the gap 36.
The above is the explanation of the developing device 19 of the
present embodiment. It should be noted that in the developing
device 19, the magnetic flux density of the gap 36 between the
outer surface of the developing sleeve 26 and the edge of the
doctor blade 35 is set to be 85 mT, which is higher than that of a
conventional developing device (less than 60 mT in a conventional
developing device). This allows the magnetic brush 40 passing
through the gap 36 in the developing device 19 to be stiffer than
that of a conventional developing device. With the developing
device 19 of the present embodiment, it is possible to prevent the
development memory which arises a problem in the conventional
developing device and a conventional image forming apparatus,
because the magnetic brush 40 passing through the gap 36 is stiffer
than that of the conventional developing device.
The following explains in detail the reasons the inventors figured
out why the development memory is generated, and then the reasons
why the developing device 19 of the present embodiment is capable
of preventing the development memory.
In a developing device, a toner film having no carrier continues to
be adhered onto the entire outer surface of a developing sleeve
having a plurality of grooves both in cases where the developing
device is in operation and not in operation, as illustrated in FIG.
7 (a). When the developing device is driven and the developing
sleeve rotates, as illustrated in FIG. 7 (b), magnetic brushes made
of carrier are adhered onto the toner film which is adhered on to
the outer surface of the developing sleeve. Further, toner powder
is attached around the magnetic brush, and the attached toner
powder is supplied to the photosensitive drum (although omitted in
the explanation of the present embodiment and the drawings such as
FIG. 6, the toner film continues to be adhered onto the entire
outer surface of the developing sleeve 26 in the developing device
19 of the present embodiment both in operation and not in
operation).
Also, a developing bias is applied to the developing sleeve while
the developing device is in operation, and each of the magnetic
brushes illustrated in FIG. 7 (b) has a substantially uniform
electric potential and all the toner powder attached around the
magnetic brushes has a substantially uniform electric
potential.
After printing an image partially having an area with extreme high
density, however, a toner film falls off from an area ".alpha." on
the outer surface of the developing sleeve that corresponds to the
area with high density (the area ".alpha." that supplied toner to
the area with high density) in the previous printing. Thus, as
illustrated in FIG. 7 (c), while an area ".beta." on the outer
surface of the developing sleeve, which is other than the area
".alpha." corresponding the area with high density, continues to
have a toner film, the area ".alpha." corresponding to the area
with high density in the previous printing is exposed.
Thereafter, when the developing sleeve in a state as illustrated in
FIG. 7 (c) is rotated for a printing process, magnetic brushes are
adhered directly onto the outer surface of the developing sleeve in
the area ".alpha.", whereas magnetic brushes are adhered onto a
toner film existing on the outer surface of the developing sleeve,
not directly onto the outer surface of the developing sleeve in the
area ".beta.".
A developing bias is applied to the developing sleeve while the
developing device is in operation. In FIG. 7 (d), magnetic brushes
are adhered directly onto the outer surface of the developing
sleeve in the area ".alpha.", whereas magnetic brushes are adhered
onto the toner film existing on the outer surface of the developing
sleeve, not directly onto the outer surface of the developing
sleeve in the area ".beta.". This causes the area ".alpha." and the
area ".beta." to have different electric potentials (The electric
potential of the magnetic brush in the area ".alpha." is lower than
that in the area ".beta."). In the toner provided from the
developing sleeve to the photosensitive drum, the toner provided
from the area ".alpha." has a lower electric potential than that
from the area ".beta.". Accordingly, in the photosensitive drum, a
part having the toner provided from the area ".alpha." has a lower
electric potential than a part having the toner provided from the
area ".beta.". As a result, in a toner image transferred from the
photosensitive drum to a sheet of paper, a part which is created by
the toner provided from the area ".alpha." has lower density than a
part which is created by the toner provided from the area
".beta.".
Namely, after printing an image partially having extremely high
density, an electric potential difference of developer is generated
between the area ".alpha." corresponding to the area printed with
high density and the area ".beta." other than the area ".alpha." in
the developing sleeve. This phenomenon is called "development
memory". When a solid image with substantially even density is
printed with the developing sleeve in which the "development
memory" is generated, the printed image has uneven density between
a part created by the toner provided from the area ".alpha." and a
part created by the toner provided from the area ".beta.".
Therefore, it is apparent that quality of the printed image is
degraded by uneven density unless the generation of the
"development memory" is prevented.
The following explains in detail how the developing device 19 of
the present embodiment can prevent the "development memory" and why
the conventional developing device is difficult to prevent the
"development memory".
As described above, the developing device 19 of the present
embodiment is designed in such a manner that the magnetic flux
density of the gap 36 between the outer surface of the developing
sleeve 26 and the edge of the doctor blade 35 is 85 mT, allowing
the magnetic brush 40 passing through the gap 36 to have higher
stiffness than a magnetic brush of the conventional developing
device.
In the developing device 19 of the present embodiment, it is
assumed that the developing sleeve 26 has an area in which a toner
film fell off from the developing sleeve 26 after printing out an
image with high density (see FIG. 7 (c)).
However, in the developing device 19 of the present embodiment, as
illustrated in FIG. 8 (a), after an area "A" in which a toner film
fell off is produced, when a doctor blade 35 and a magnetic brush
40 adhered onto a toner film 51 in an area "B" collide with each
other, there is a short time lag between (i) the timing of
collision and (ii) the timing when the magnetic brush 40 is broken
by the collision because the magnetic brush 40 has higher stiffness
than a magnetic brush of the conventional developing device. During
this time lag, the impact generated by the collision between the
magnetic brush 40 and the doctor blade 35 is transmitted to the
contact point between the magnetic brush 40 and the toner film 51.
This transmitted impact breaks a part of the toner film 51 into
toner powder. The toner powder is carried from the area "B" to the
area "A" by the rotation of the developing sleeve 26. As a result,
a new toner film is created in the area "A". As this process
illustrated in FIG. 8 continues, a toner film is recreated entirely
on the outer surface of the developing sleeve 26. Therefore, even
if there is temporarily a part in which a toner film fell off from
the outer surface of the developing sleeve 26 after printing an
image with high density, the developing device 19 of the present
embodiment can immediately recreate a new toner film on the part.
This allows a toner film to be formed substantially constantly on
the entire outer surface of the developing sleeve 26 so as not to
maintain the situations illustrated in FIG. 7 (c) and FIG. 7 (d).
As a result, it is possible to prevent the generation of the
development memory and the degradation of the printed image.
On the other hand, according to the conventional developing device,
as illustrated in FIG. 8 (b), when a doctor blade and a magnetic
brush adhered onto a toner film in an area "D" are collided with
each other by the rotation of the developing sleeve, the magnetic
brush is immediately broken by the collision impact because the
magnetic brush is less stiff than that of the present embodiment
and the collision impact is absorbed by the break of the magnetic
brush. Accordingly, the collision impact generated between the
magnetic brush and the doctor blade is less apt to be transmitted
to the contact part between the magnetic brush and the toner film.
This rarely causes a break of the toner film. Accordingly, the
toner in the area "D" is never transmitted to an area "C" which
does not have a toner film. Therefore, unlike the developing device
19 of the present embodiment, once the area "C" which has no toner
film is created on the developing sleeve by printing an image with
high density, the conventional developing device is not able to
recreate a new toner film at the area "C" immediately and tends to
have the situations illustrated in FIG. 7 (c) and FIG. 7 (d)
continuously. This causes the development memory, which leads to
the image degradation.
Also, with the developing device 19 of the present embodiment, the
magnetic flux density of the gap 36 is set to be 85 mT. However, it
is not limited to this value. The following examples of experiments
show that it is possible to suppress the development memory when
the magnetic flux density of the gap 36 ranges from 70 mT to 150
mT. The magnetic flux density of the gap 36 is required to be 70 mT
or more in order to increase the stiffness of the magnetic brush 40
to suppress the development memory. On the other hand, the magnetic
flux density of the gap 36 is required to be 150 mT or less for the
reason as follows. If the magnetic flux density of the gap 36
exceeds 150 mT, the magnetic brush 40 becomes too stiff, and the
absorbability between the magnetic brush 40 and the toner film 51
in FIG. 8 (a) becomes too strong. Consequently, the toner film 51
in the area "B" becomes hard to be broken by the magnetic brush 40.
This makes it more difficult to recreate a new toner film in the
area "A", resulting in that the development memory tends to be
generated.
Further, the outer surface of the developing sleeve 26 has a
plurality of grooves. As illustrated in FIG. 5, the grooves are
designed to have a rectangular shape (substantially rectangular) in
a cross section of the developing sleeve 26 when it is cut by a
plane perpendicular to the axis of the developing sleeve 26. This
shape helps to prevent accumulating developer inside the grooves.
When the toner film 51 in the area "B" is scraped due to the high
stiffness of the magnetic brush 40 and toner powder is produced,
the toner powder is carried easily from the area "B" with a toner
film 51 to the area "A" with no toner film because toner powder is
less likely to be accumulated in the grooves between the area "A"
and the area "B". This allows suppressing the generation of the
development memory.
Further, according to the later-mentioned experiment examples, it
is preferable that a pitch between grooves provided on the outer
surface of the developing sleeve 26 ranges from 1.25 mm to 4.00 mm.
The following explains the reasons why the pitch should range from
1.25 mm to 4.00 mm. In a case where the number of grooves provided
on the outer surface of the developing sleeve 26 is too large,
there is a possibility that when the toner film 51 in the area "B"
is scraped due to the high stiffness of the magnetic brush 40 and
toner powder is produced, a part of the toner powder is not
smoothly carried from the area "B" to the area "A" due to the too
large number of the grooves. When the pitch between grooves is 1.25
mm or more, the number of grooves is considered not so large as to
negatively affect the carriage of the toner powder. This allows
smooth carriage of the toner powder from the area "B" to the area
"A", resulting in that the generation of the development memory is
prevented effectively. When the pitch between grooves exceeds 4.00
mm, the number of grooves becomes too small, which makes it
difficult to achieve the original purpose for providing grooves on
the outer surface of the developing sleeve 26 (i.e. improving the
magnetic attraction for developer stored inside the developer tank
27 to suppress generation of an image with low density). In the
present embodiment, the pitch between grooves means the distance
between a side wall of a groove S located at the downstream with
regard to the direction T and a side wall of a groove Q located at
the downstream with regard to the direction T as illustrated in
FIG. 5.
Further, according to the later-mentioned experiment examples, it
is preferable that the width of each groove on the developing
sleeve 26 ranges from 0.20 mm to 0.35 mm. The following explains
the reason why the width of the groove should range from 0.20 mm to
0.35 mm. When the width of the groove on the developing sleeve 26
is less than 0.2 mm, the groove is too small, which makes it
difficult to achieve the original purpose for providing the groove
on the developing sleeve 26 (i.e. improving the magnetic attraction
for developer stored inside the developer tank 27 to suppress
generation of an image with low density), and makes it difficult to
obtain stable image density under high temperature and high
humidity conditions in particular. On the other hand, when the
width of the groove on the developing sleeve 26 exceeds 0.35 mm,
unevenness in image density occurs. In the present embodiment, the
width of the groove means the length of the bottom plane of the
groove in the direction T as illustrated in FIG. 5.
Further, according to the later-mentioned experiment examples, it
is preferable that the depth of the groove on the developing sleeve
26 ranges from 0.08 mm to 5.00 mm. The following explains the
reason why the depth of the groove should range from 0.08 mm to
5.00 mm. When the depth of the groove is less than 0.08 mm, the
groove is too small, which makes it difficult to achieve the
original purpose for providing the groove on the developing sleeve
26 (i.e. improving the magnetic attraction for developer stored
inside the developer tank 27 to suppress generation of an image
with low density). When the depth of the groove exceeds 5 mm,
developer is likely to be accumulated inside the groove in
long-term usage, which causes the degradation of a printed image.
In the present embodiment, the depth of the groove means the
distance between the outer surface of the developing sleeve 26 and
the bottom of the groove in the diameter direction.
In the developing device 19 of the present embodiment, as
illustrated in FIG. 4, the magnet S1 for generating magnetism at
the gap 36 is provided between the rotation axis of the developing
sleeve 26 and the doctor blade 35. In other words, the magnet S1 is
located closer to the gap 36 inside the developing sleeve 26. This
allows the magnetism to work effectively at the gap 36, making it
easy to set the magnetic flux density at the gap 36 to range from
70 mT to 150 mT.
Further, in the present embodiment, the doctor blade 35 may be made
of any one of magnetic metal, non-magnetic metal, and plastic. If
the doctor blade 35 is made of magnetic metal, the gap 36 is
sandwiched between the magnet S1 and the magnetic material.
Consequently, the synergetic effect of the magnet S1 and the
magnetic material works, which makes it more easy to set the
magnetic flux density at the gap 36 to be 70 mT or more. Examples
of the magnetic material include: stainless JIS 400's (SUS 410, SUS
420, and SUS 430), and stainless JIS 329's (SUS 329).
Further, in the developing device 19 of the present embodiment,
when a volume average particle size of carrier in developer is less
than 20 .mu.m, the magnetic brush 40 illustrated in FIG. 8 (a)
becomes less stiff and less apt to scrape off the toner film
51.
On the other hand, when the volume average particle size of carrier
in developer exceeds 60 .mu.m, the specific surface area of carrier
becomes small. This is likely to causes fogs and undesirable
density of an image because the developer has narrower allowable
range of toner density. Namely, when the volume average particle
size of carrier becomes too large, the specific surface area of
carrier becomes small. As a result, the coating ratio of toner
particles to carrier increases, which drops the amount of toner
attracted by the developing roller 24 (There is no space for new
toner particles on the surface of the carrier particles entirely
covered by toner particles).
Therefore, in the developing device 19 of the present embodiment,
it is preferable that the volume average particle size of carrier
ranges from 20 .mu.m to 60 .mu.m in order to suppress fogs,
undesirable density in image, and the generation of the development
memory.
Further, in the developing sleeve 19 of the present embodiment, it
is preferable to use the carrier obtained by covering ferrite-based
core particles with thermosetting resin. The breakage of carrier
can be prevented by using the carrier covered by the thermosetting
resin since the resin covering the carrier is hard to be peeled off
even when the carrier in the magnetic blade 40 continuously scrapes
off the toner film 51 as illustrated in FIG. 8 (a).
Further, in the developing sleeve 19 of the present embodiment, it
is preferable to use the carrier whose saturated magnetization
ranges from 30 emu/g to 70 emu/g. This makes the magnetic brush 40
as illustrated in FIG. 8 (a) stiff enough to scrape off the toner
film 51, which allows preventing the generation of the development
memory further.
Recently, small particle toner whose volume average particle size
ranges from 4 .mu.m to 7 .mu.m has been widely used in order to
improve the quality of a printed image. As the particle size of
toner gets smaller, the phenomenon illustrated in FIG. 7 (a)
through FIG. 7 (d), i.e. the development memory is more likely to
occur. Thus, the developing device 19 of the present embodiment is
preferably applicable to the image forming apparatus using toner
whose volume average particle size ranges from 4 .mu.m to 7
.mu.m.
EXPERIMENT EXAMPLES
The inventors carried out printing tests (comparison tests) by
using image forming apparatuses of Examples 1 though 19 and image
forming apparatuses of Comparative Examples 1 thorough 13. The
following explains Examples 1 through 19, Comparative Examples 1
through 13, and the results of the printing tests.
Example 1
Example 1 was the image forming apparatus 50 of the present
embodiment illustrated in FIG. 2 through FIG. 5. In the black image
forming unit 1, a photosensitive drum 16 of 60 mm in diameter and a
developing roller 24 of 40 mm in diameter were used. In each of the
image forming units 2 through 4 other than the black image forming
unit 1, a photosensitive drum of 30 mm in diameter and a developing
roller 24 of 20 mm in diameter were used. The process velocity of
each of the image forming units 1 through 4 was set to be 175
mm/sec., and the peripheral velocity of the developing roller was
set to be 280 mm/sec. Further, in the image forming apparatus 50 of
the present embodiment, each image forming unit had a developing
sleeve having on its outer surface a plurality of grooves each
being 2.0 mm in pitch, 0.25 mm in width, and 1.0 mm in depth.
Further, each of the image forming units 1 through 4 was adjusted
so that the amount of toner deposited on a sheet of paper which was
provided from each image forming unit was 0.5 mg/cm.sup.2. There
was conducted a printing test to print a chart in which a halftone
image whose image density ID ranged from 0.5 to 0.8 was printed as
a background, and a solid image part (1 cm.times.1 cm) colored by
respective colored toners whose image density IDs were 1.4 or more
were located at the head part of the chart in the paper feeding
direction.
Note that the image density ID is an image density measured by
using a Macbeth densitometer and is represented by the following
equation. ID=10log(Pi/Po)
where Pi is intensity of light incident to an image and Po is
intensity of light reflected from an image.
As a result of the printing test to print the chart by the image
forming apparatus 50 of Example 1, the printed chart was clear. As
shown in Table 1, unevenness in density due to the development
memory, abnormal density due to insufficient attraction of
developer, image degradation caused by other reasons, and clogged
grooves of a developing sleeve were not detected.
TABLE-US-00001 TABLE 1 Grooves on the Evaluation Result developing
sleeve Magnetic Insufficient width pitch depth flux Development
attraction of Other image (mm) (mm) (mm) density * Memory developer
degradation Remarks EX. 1 0.2 2 1 85 N/A N/A N/A EX. 2 0.25 2 1 85
N/A N/A N/A EX. 3 0.3 2 1 85 N/A N/A N/A EX. 4 0.35 2 1 85 N/A N/A
N/A EX. 5 0.25 1.25 1 85 N/A N/A N/A EX. 6 0.25 1.5 1 85 N/A N/A
N/A EX. 7 0.25 2 1 85 N/A N/A N/A EX. 8 0.25 3 1 85 N/A N/A N/A EX.
9 0.25 4 1 85 N/A N/A N/A EX. 10 0.25 2 0.08 85 N/A N/A N/A EX. 11
0.25 2 0.16 85 N/A N/A N/A EX. 12 0.25 2 1 85 N/A N/A N/A EX. 13
0.25 2 2 85 N/A N/A N/A EX. 14 0.25 2 5 85 N/A N/A N/A EX. 15 0.25
2 1 70 N/A N/A N/A EX. 16 0.25 2 1 85 N/A N/A N/A EX. 17 0.25 2 1
100 N/A N/A N/A EX. 18 0.25 2 1 120 N/A N/A N/A EX. 19 0.25 2 1 150
N/A N/A N/A * Magnetic flux density between developing sleeve and
doctor blade (mT)
TABLE-US-00002 TABLE 2 Grooves on the Evaluation Result developing
sleeve Magnetic Insufficient width pitch depth flux Development
attraction of Other Image (mm) (mm) (mm) density * Memory developer
degradations Remarks Com. EX. 1 0.1 2 1 85 N/A detected N/A Com.
EX. 2 0.15 2 1 85 N/A detected N/A Com. EX. 3 0.4 2 1 85 N/A N/A
detected Com. EX. 4 0.45 2 1 85 N/A N/A detected Com. EX. 5 0.25 5
1 85 N/A detected N/A Com. EX. 6 0.25 6 1 85 N/A detected N/A Com.
EX. 7 0.25 2 0.02 85 N/A detected N/A Com. EX. 8 0.25 2 0.05 85 N/A
detected N/A Com. EX. 9 0.25 2 7 85 N/A N/A N/A grooves were
clogged Com. EX. 10 0.25 2 10 85 N/A N/A N/A grooves were clogged
Com. EX. 11 0.25 2 1 50 detected N/A N/A Com. EX. 12 0.25 2 1 60
detected N/A N/A Com. EX. 13 0.25 2 1 180 detected N/A N/A *
Magnetic flux density between developing sleeve and doctor blade
(mT)
Examples 2 through 19
Image forming apparatuses 50 of Examples 2 through 19 were
basically the same as the image forming apparatus of Example 1
except that only the pitch between grooves on the outer surface of
the developing sleeve, the width of the groove, the depth of the
groove, and the magnetic flux density between the developing sleeve
and the doctor blade were changed to the values mentioned in Table
1. The printing tests were carried out with respect to the image
forming apparatuses of Examples 2 though 19 under the same testing
condition as that of the Example 1. As a result, the obtained
charts were printed clearly as mentioned in Table 1. The unevenness
in density due to the development memory, the abnormal density due
to insufficient attraction of developer, the image degradation
caused by other reasons, and clogged grooves of the developing
sleeve were not detected.
Comparative Examples 1 through 13
Image forming apparatuses of Comparative Examples through 13 were
basically the same as the image forming apparatus of Example 1
except that only the pitch between grooves on the outer surface of
the developing sleeve, the width of the groove, the depth of the
groove, and the magnetic flux density between the developing sleeve
and the doctor blade were changed to the values mentioned in Table
2. The printing tests were carried out in the image forming
apparatuses of Comparative Examples 1 though 13 under the same
testing condition as that of Example 1. As a result, as shown in
Table 2, one of unevenness in density due to the development
memory, abnormal density due to insufficient attraction of
developer, image degradation caused by other reasons, and clogged
grooves of the developing sleeve was detected.
Considering the test results of Examples 1 through 19 and
Comparative Examples 1 through 13, it is clear that when the
magnetic flux density ranged from 70 mT to 150 mT, no development
memory was generated.
Also, considering the test results of Examples 1 through 19 and
Comparative Examples 7 through 10, insufficient attraction of
developer was detected when the depth of the groove on the outer
surface of the developing sleeve was less than 0.08 mm, and the
clogging of developer occurred in the groove on the outer surface
of the developing sleeve when the depth of the groove exceeded 5
mm.
Further, considering the test results of Examples 1 through 19 and
Comparative Examples 1 through 4, insufficient attraction of
developer was detected when the width of the groove was less than
0.2 mm, and image degradation caused by a reason other than the
development memory and the insufficient attraction of developer was
detected when the width of the groove exceeded 0.35 mm.
[Test Condition]
The following explains in detail the printing test condition under
which Examples 1 though 19 and Comparative Examples 1 through 13
were carried out.
(a) Paper
Recycled A4 size paper (Recycle Pure: SHARP DOCUMENT SYSTEMS
CORPORATION) was used for the printing tests.
(b) Toner
The following explains in detail the manufacturing method of the
toner contained in the two-component developer which was used in
the printing tests.
First, 100 parts by weight of binder resin, 5 parts by weight of a
coloring agent, 2 parts by weight of a charge control agent, 3
parts by weight of a release agent were put in a Henschel mixer as
raw materials, and were mixed for 10 minutes. Then, the obtained
mixture was melted, kneaded, and dispersed by a kneading and
dispersion machine (KNEADEX MOS 140-800 manufactured by MITSUI
MINING COMPANY, LIMITED).
As a binder resin, polyester resin (glass-transition temperature is
60.degree. C.; softening temperature is 120.degree. C.), which was
obtained by polycondensation of bisphenol A monomer, propylene
oxide monomer, and terephthalic acid monomer was used. Also, it is
possible to use trimellitic anhydride instead of terephthalic
acid.
Further, as a coloring agent for black toner, carbon black (MA-1
manufactured by Mitsubishi Chemical Corporation) was used; C. I.
pigment red 122 was used for magenta toner; C. I. pigment blue 15:3
was used for cyan toner; and C. I. pigment yellow 74 was used for
yellow toner. Also, as a charge control agent, a salicylate zinc
compound (BONTRON E84 manufactured by Orient Chemical Industries,
Ltd.) was used. As a release agent, a microcrystalline wax (HNP-9
manufactured by NIPPON SEIRO CO., LTD.) was used.
The mixture obtained by the melting, kneading and dispersing
process was roughly pulverized by a cutting mill, and then was
finely pulverized by a jet pulverizer (IDS-2 manufactured by Nippon
Pneumatic Mfg. Co., Ltd.). The finely pulverized mixture was
classified by a wind classifier (MP-250 manufactured by Nippon
Pneumatic Mfg. Co., Ltd.). As a result, color resin particles whose
volume average particle size was 6.5 .mu.m were obtained. The
volume average particle size was measured by a Coulter Multisizer 2
(manufactured by Beckman Coulter K.K.).
Further, 1.5 parts by weight of hydrophobic silica fine particles
of approximately 12 nm in average primary particle size that were
surface-treated with hexamethyldisilazane (BET specific surface
area was approximately 140 m.sup.2/g) and 1.0 parts by weight of
hydrophobic silica fine particles of approximately 40 nm in average
primary particle size that were surface-treated in the same way
(BET specific surface area was approximately 50 m.sup.2/g) were
added to 100 parts by weight of the obtained color resin particles,
and the resultant was mixed by Henschel mixer for two minutes to
create negatively charged toner.
(c) Carrier
The following explains in detail the manufacturing method of the
carrier contained in the two-component developer which was used in
the printing tests.
First, ferrite powder was measured and was mixed by a ball mill.
The mixture was calcinated by a rotary kiln at 900.degree. C. The
calcinated ferrite powder was finely pulverized by a wet pulverizer
to obtain ferrite fine powder of 2 .mu.m or less in average
particle size with use of steal balls as pulverizing media. The
obtained ferrite fine powder was granulated in a spray dry method
to be fine powder of 100 .mu.m to 200 .mu.m in particle size. The
granulated substance was sintered at 1300.degree. C., and was
crushed by a crusher to obtain ferrite particles of approximately
35 .mu.m in volume average particle size. The ferrite particles
were used as core particles of the carrier.
A coating liquid for coating the core particles was obtained by
dissolving or dispersing silicone resin (TSR115 manufactured by
Sin-Etsu Chemical Co., Ltd.) and potassium titanate powder
(resistance adjusting agent) in toluene.
The ferrite particles were coated with the coating liquid by use of
a spray coating machine, and was dried naturally to remove toluene.
Thus was obtained carrier of 55 emu/g in saturated magnetization
and 35 .mu.m in volume average particle size, coated by 5 weight %
of silicone resin (having 5 weight % of silicone resin in density).
Note that the coating amount of silicone resin was calculated based
on the amount of Fe derived from the ferrite particles and the
amount of Si derived from the silicone resin, both of which amounts
were measured by a fluorescent X-ray analyzer.
(d) Two-Component Developer
Two-component developers including respective color toners (black,
cyan, magenta, and yellow) were produced by stirring and mixing 5
parts by weight of the respective color toners obtained in the
above manner and 95 parts by weight of carrier with a Nauta mixer
(VL-0 manufactured by Hosokawa Micron Corporation) for 20 minutes.
The obtained two-component developers were used in the printing
test.
(e) Photosensitive Drum
The photosensitive drums (layered photosensitive drum), which were
used in Examples 1 through 19 and Comparative Examples 1 through
13, were manufactured through the following method.
First, 7 parts by weight of titanium oxide (TTO55A manufactured by
ISHIHARA SANGYO KAISHA LTD.) and parts by weight of copolymer nylon
(CM8000 manufactured by TORAY INDUSTRIES, INC) were added to a
mixed solvent of 159 parts by weight of methyl alcohol and 106
parts by weight of 1,3-dioxolane. Additives in the mixed solvent
were dispersed with a paint shaker for 8 hours to obtain a coating
liquid for forming an underlying layer.
A cylindrical aluminum conductive base substance was soaked in a
coating tank filled with the coating liquid for an underlying layer
so that the liquid was applied on the surface of the cylindrical
aluminum conductive base substance. Thereafter, the cylindrical
aluminum conductive base substance was taken out from the tank, and
was dried naturally to form an underlying layer of 1 .mu.m in
thickness on the peripheral surface of the cylindrical aluminum
conductive base substance.
Next, 3 parts by weight of titanylphthalocyanine and 2 parts by
weight of butyral resin (BL-1 manufactured by SEKISUI CHEMICAL CO.,
LTD) were added to 245 parts by weight of methylethylketone. The
additives were dispersed with a paint shaker to obtain a coating
liquid for forming a charge generating layer.
The coating liquid for forming a charge generating layer was
applied on the surface of the underlying layer through the same
soaking and applying method as that used when forming the
underlying layer, and was dried naturally without wiping out the
lower end. As a result, a charge generating layer of 0.4 .mu.m in
thickness was formed on the underlying layer of the cylindrical
aluminum conductive base substance.
Next, 5 parts by weight of a charge transport compound (T405
manufactured by Takasago Chemical Corp.), 2.4 parts by weight of
polycarbonate (J500 manufactured by Idemitsu Kosan Co., Ltd.), 1.6
parts by weight of polycarbonate (G400 manufactured by Idemitsu
Kosan Co., Ltd.), 1.6 parts by weight of polycarbonate (GHSO.sub.3
manufactured by Idemitsu Kosan Co., Ltd.), 2.4 parts by weight of
polycarbonate (TS2020 manufactured by TEIJIN CHEMICALS LTD.), and
0.25 parts by weight of 2,6-bis-tert-butyl-4-methylphenol
(Sumilizer BHT manufactured by Sumitomo Chemical Co., Ltd.) were
added to and melted in 49 parts by weight of tetrahydrofuran. As a
result, a coating liquid for forming a charge transport layer was
obtained. The cylindrical aluminum conductive base substance was
soaked in a coating tank filled with the coating liquid for forming
a charge transport layer so that the liquid was applied on the
surface of the charge generating layer through the soaking and
applying method. Thereafter, the cylindrical aluminum conductive
base substance was taken out from the tank and dried at 130.degree.
C. for 1 hour so that the charge transport layer was formed on the
charge generating layer. Thus, an electro photoreceptor of 25 .mu.m
in thickness was obtained. Note that the thickness of the
photosensitive drum was measured by a spectrophotometer (MCPD-1100
manufactured by OTSUKA ELECTRONICS CO., LTD).
[Details of Two-Component Developer]
The following explains in more detail the two-component developer
used in the image forming apparatus 50 of the present
embodiment.
The two-component developer of the present embodiment was made by
mixing the toner and the carrier by a mixer such as a Nauta mixer.
Normally, the ratio of toner to be mixed with carrier ranges from 3
to 15 parts by weight of toner with respect to 100 parts by weight
of carrier. The toner and the carrier which are used in the present
embodiment are explained below in this order.
The toner used in the present embodiments is made by adding an
external additive to the surfaces of colored resin particles with
an air flow mixer such as a Henschel mixer. The colored resin
particles are made through publicly known methods such as a
kneading and crushing method, or a polymerization. In the kneading
and crushing method as an example, binder resin, a boron compound,
a coloring agent, and other additives are mixed by a mixer such as
Henschel mixer, a super mixer, a MECHANOMILL, and a Q-type mixer.
The obtained material mixture was melted and kneaded by a kneader
such as a biaxial kneader and a uniaxial kneader at approximately
70 to 180.degree. C. The resultant thus kneaded was solidified by
cooling, and then the solidified substance was pulverized by an air
pulverizer such as a jet mill. If needed, size control such as
classification may be conducted.
It is preferable that the volume average particle size of the
colored resin particles (toner) ranges from 4 .mu.m to 7 .mu.m,
which is measured by a Coulter Counter with 100 .mu.m aperture
manufactured by Coulter Corporation. When the volume average
particle size is less than 4 .mu.m, electric charging amount is
unstable due to low flowability of the toner. When the volume
average particle size exceeds 7 .mu.m, dot reproducibility
drops.
As a binder resin contained in the toner of the present embodiment,
publicly known styrene acrylic resin and polyester resin may be
used. Specifically, linear or nonlinear polyester resin is
preferable because polyester resin is excellent in mechanical
strength (less likely to produce fine powder), a fixing property
(less likely to be detached from the paper after fixation), and
anti-hot-offset property.
The polyester resin is obtained by polymerizing multivalent alcohol
(bivalent or higher) and polybasic acid (bivalent or higher).
Alternatively, if needed, the polyester resin is obtained by
polymerizing a monomer composition including multivalent alcohol
(tervalent or higher) or polybasic acid (tervalent or higher).
Examples of bivalent alcohol used for polymerization to produce
polyester resin include: glycols such as ethylene glycol,
diethylene glycol, triethylene glycol, 1,2-propylene glycol,
1,3-propylene glycol, 1,4-butanediol, and neopentyl glycol; diols
such as 1,4-butendiol, 1,5-pentanediol, and 1,6-hexanediol;
bisphenol A alkylene oxide adducts such as bisphenol A,
hydrogenated bisphenol A, polyoxyethylenated bisphenol A, and
polyoxypropylenated bisphenol A.
Examples of multivalent alcohol (tervalent or higher) include:
sorbitol, 1,2,3,6-hexanetetrol, 1,4-sorbitan, pentaerythritol,
dipentaerythritol, tripentaerythritol, saccharose,
1,2,4-butanetriol, 1,2,5-pentanetriol, glycerol,
2-methylpropantriol, 2-methyl-1,2,4-butanetriol,
trimethylolmethane, trimethylolpropane, 1,3,5-trihydroxy
methylbenzene, and the like.
Examples of bivalent polybasic acid include: maleic acid, fumaric
acid, citraconic acid, itaconic acid, glutaconic acid, phthalic
acid, isophthalic acid, terephthalic acid, cyclohexanedicarboxylic
acid, succinic acid, adipic acid, sebacic acid, azelaic acid,
malonic acid, anhydrides of the forgoing acids, lower alkylester,
alkenylsuccinic acids such as n-dodecynylsuccinic acid and
n-dodecylsuccinic acid, and alkylsuccinic acids.
Examples of multivalent alcohol (tervalent or higher) include:
1,2,4-benzene tricarboxylic acid, 1,2,5-benzene tricarboxylic acid,
1,2,4-cyclohexane tricarboxylic acid, 2,5,7-naphthalene
tricarboxylic acid, 1,2,4-naphthalene tricarboxylic acid,
1,2,5-hexane tricarboxylic acid,
1,3-dicarboxylic-2-methyl-2-methylene carboxy propane, tetra
(methylenecarboxyl) metan, 1,2,7,8-octane tetracarboxylic acid, and
anhydrides of the forgoing acids.
As the coloring agent which is used in the toner used in the image
forming apparatus 50 of the present embodiment, generally-used and
publicly known pigment and dye can be used. Specifically, examples
of the coloring agent for black toner include: carbon black and
magnetite.
Examples of the coloring agent for yellow toner include:
acetoacetic acid arylamide monoazo yellow pigments such as C.I.
pigment yellow 1, C.I. pigment yellow 3, C.I. pigment yellow 74,
C.I. pigment yellow 97, C.I. and pigment yellow 98; acetoacetic
acid arylamide disazo yellow pigments such as C.I. pigment yellow
12, C.I. pigment yellow 13, C.I. pigment yellow 14, and C.I.
pigment yellow 17; condensed monoazo yellow pigments such as C.I.
pigment yellow 93 and C.I. pigment yellow 155; other yellow
pigments such as C.I. pigment yellow 180, C.I. pigment yellow 150,
and C.I. pigment yellow 185; yellow dyes such as C.I. solvent
yellow 19, C.I. solvent yellow 77, C.I. solvent yellow 79, and C.I.
disperse yellow 164.
Examples of the coloring agent for magenta toner include: red or
sanguine pigments such as C.I. pigment red 48, C.I. pigment red
49:1, C.I. pigment red 53:1, C.I. pigment red 57, C.I. pigment red
57:1, C.I. pigment red 81, C.I. pigment red 122, C.I. pigment red
5, C.I. pigment red 146, C.I. pigment red 184, C.I. pigment red
238, and C.I. pigment violet 19; reddish dyes such as C.I. solvent
red 49, C.I. solvent red 52, C.I. solvent red 58, and C.I. solvent
red 8.
Examples of the coloring agent for cyan toner include: bluish
pigments and dyes made of copper phthalocyanine and its derivatives
such as C.I. pigment blue 15:3 and C.I. pigment blue 15:4; green
pigments such as C.I. pigment green 7 and C.I. pigment 36
(phthalocyanine green). The amount of the coloring agent to be
added preferably ranges from 1 to 15 parts by weight, and more
preferably ranges from 2 to 10 parts by weight, with respect to 100
parts by weight of binder resin.
For the toner used in the image forming apparatus 50 of the present
embodiment, a publicly known charge control agent can be used.
Specifically, examples of the charge control agent providing
negative charge include: chromeazo complex dye, ironazo complex
dye, cobaltazo complex dye, chromium/zinc/aluminum/boron complex
with salicylic acid or its derivative, or salt compounds of
salicylic acid or its derivative; chromium/zinc/aluminum/boron
complex with naphthol acid or its derivative, or salt compounds of
naphthol acid or its derivative; chromium/zinc/aluminum/boron
complex with benzilic acid or its derivative, or salt compounds of
benzilic acid or its derivative; long-chain alkyl carboxylate, and
long-chain alkyl sulfonate.
Examples of a charge control agent providing positive charge
include: nigrosine dye and its derivative, triphenylmethane
derivative, quaternary ammonium salt derivative, quaternary
phosphonium salt derivative, quaternary pyridinium salt derivative,
guanidine salt derivative, and amidin salt derivative.
The amount of the charge control agent to be added preferably
ranges from 0.1 to 20 parts by weight, more preferably ranges from
0.5 to 10 parts by weight, with respect to 100 parts by weight of
binder resin.
Examples of the release agent, which is used with the toner for the
image forming apparatus 50 of the present embodiment, include:
synthetic waxes such as polypropylene and polyethylene; petroleum
waxes and its denatured wax such as paraffin wax and its derivative
and microcrystalline wax and its derivative; botanical waxes such
as carnauba wax, rice wax, and candelilla wax. It is possible to
increase the releasability of toner from a fixing roller and a
fixing belt by adding these release agents to the toner, thereby
preventing high-temperature and low-temperature offset in a fixing
process.
A publicly known fluidizer may be added to the toner which is used
for the image forming apparatus 50 of the present embodiment for
the purpose of improving the flowability of the toner. Examples of
the available fluidizers include hydrophobized inorganic particles
obtained by surface-treating silica, titanium oxide, alumina, and
the like of 0.007 to 0.03 .mu.m in average particle size with use
of a silane coupling agent, a titan coupling agent, or silicone
oil.
It is preferable that the amount of the fluidizer to be added
ranges from 0.3 to 3 parts by weight with respect to 100 parts by
weight of toner. It is impossible to obtain the effect of the
fluidizer, when the amount of the fluidizer to be added is less
than 0.3 parts by weight. On the other hand, when the amount is not
less than 3 parts by weight, flowability is likely to drop.
The carrier used in the image forming apparatus 50 may be magnetic
particles from 20 to 60 .mu.m in weight average particle size,
obtained by coating the surface of magnetic core particles with a
coating material. When the particle size of the carrier is less
than 20 .mu.m, an obtained image has white spots because the
carrier is transmitted from the developing roller 24 to the
photosensitive drum 16 in a developing process. On the other hand,
when the particle diameter of the carrier exceeds 60 .mu.m, an
obtained image becomes rough due to drop in dot reproducibility.
The volume average particle size of the carrier was measured by a
laser diffraction particle size analyzer HELOS (made by Sympatec
GmbH) in combination with a dry disperser RODOS (made by Sympatec
GmbH) under dispersive pressure 3.0 bar.
It is preferable that the saturated magnetization of carrier ranges
from 30 emu/g to 70 emu/g. When the saturated magnetization of
carrier exceeds 70 emu/g, a magnetic brush becomes stiff, which
makes it difficult to obtain an image true to an electrostatic
latent image, and which makes it more likely that white spots
appear. On the other hand, as the saturated magnetization of
carrier is lower, the magnetic brush in contact with a
photosensitive drum becomes softer. This allows obtaining an image
true to an electrostatic latent image. However, when the saturated
magnetization is less than 30 emu/g, the magnetic brush becomes too
soft. This tends to cause the development memory because the
magnetic brush fails to scrape off a toner film. Also, white spots
are likely to appear due to the carrier adhesion on the surface of
the photosensitive drum.
As core particles of carrier, publicly known magnetic particles can
be used. It is preferable to use ferrite particles in terms of
charging property and endurance. Publicly known ferrite particles
can be used. Examples of the ferrite particles include: zinc
ferrite, nickel ferrite, copper ferrite, nickel-zinc ferrite,
manganese-magnesium ferrite, copper-magnesium ferrite,
manganese-zinc ferrite, and manganese-copper-zinc ferrite. These
ferrite particles are obtained by mixing materials, calcining and
pulverizing them, and thereafter sintering them. Also, it is
possible to alter the surface shape of ferrite particles by
changing sintering temperature. Note that calcification may be
carried out with a batch type machine, or a continuous type such as
a rotary kiln.
As a coating agent, publicly known resin materials can be used.
However, it is particularly preferable to use thermosetting resin
in terms of resistance to abrasion. Publicly known thermosetting
resins can be used. Examples of the thermosetting resins include:
silicone varnishes such as TSR 115, TSR 114, TSR 102, TSR 103, YR
3061, TSR 110, TSR 116, TSR 117, TSR 108, TSR 109, TSR 180, TSR
181, TSR 187, TSR 144, and TSR 165 (made by Toshiba Corporation),
KR 271, KR 272, KR 275, KR 280, KR 282, KR 267, KR 269, KR 211, and
KR 212 (made by Shin-Etsu Chemical Co., Ltd.); alkyd denatured
silicone varnishes such as TSR 184, and TSR 185 (made by Toshiba
Corporation); epoxy denatured silicone varnishes such as TSR 194,
and YS 54 (made by Toshiba Corporation); polyester denatured
silicone varnishes such as TSR 187 (made by Toshiba Corporation);
acrylic denatured silicone varnishes such as TSR 170, and TSR 171
(made by Toshiba Corporation); urethane denatured silicone
varnishes such as TSR 175 (made by Toshiba Corporation); reactive
silicone resins such as KA 1008, KBE 1003, KBC 1003, KBM 303, KBM
403, KBM 503, KBM 602, and KBM 603 (made by Shin-Etsu Chemical Co.,
Ltd.).
It is preferable that a resistance adjuster is added to the coating
agent in order to control the resistance of carrier. Specifically,
examples of the resistance adjuster include: silicon oxide,
alumina, carbon black, graphite, zinc oxide, titan black, iron
oxide, titanium oxide, tin oxide, potassium titanate, calcium
titanate, aluminum borate, magnesium oxide, barium sulfate, and
calcium oxide.
A publicly known method can be used in order to coat the carrier
particles with a coating agent. Examples of the method include: a
method in which the carrier particles are soaked in an organic
solvent of the coating agent, a spray method in which the organic
solvent of the coating agent is sprayed to the carrier particles, a
fluid bed method in which the organic solvent of the coating agent
is sprayed to the carrier particles floating in the fluidized air,
a kneader coater method in which the carrier particles and the
organic solvent of the coating agent are mixed with each other in a
kneader coater, and then the resulting solvent is evaporated off.
In this procedure, the resistance adjuster may be added to the
organic solvent of the coating agent.
[Details of the Photosensitive Drum]
The following explains in detail the photosensitive drum used in
the image forming apparatus 50 of the present embodiment.
Examples of the photosensitive drum 16 included in the image
forming apparatus 50 of the present embodiment include an organic
cylindrical photosensitive drum including a conductive base
substance and a photosensitive layer, and an amorphous silicon
photosensitive drum. Among them, the organic photosensitive drum is
preferable in terms of manufacturing cost and safety. Organic
photosensitive drums are classified into two types: a multi-layered
type and a single-layered type. The multi-layered type is
preferable since it is excellent in sensitivity and residual
potential. The multi-layered photosensitive drum includes: a
conductive base substance; a charge generating layer having a
charge generating compound, layered on the base substance; and a
charge transport layer having a charge transport compound, layered
on the charge generating layer. It is more preferable to have an
underlying layer between the conductive base substance and the
charge generating layer.
As a conductive base substrate, for example, a cylindrical shaped
aluminum, a cylindrical shaped plastic including conductive
particles, and the like are available. Examples of the underlying
layer include polyamide resin and copolymerized nylon resin in
which an inorganic pigment such as zinc oxide and titanium oxide is
dispersed with a disperser such as a ball mill and a dyno mill.
The charge generating layer is a charge generating substance which
generates electrical charge by light radiation. Examples of the
charge generating layer include polycarbonate resin, phenoxy resin,
phenol resin, polyvinylbutyral resin, polyalylate resin, polyamide
resin, and polyester resin, in each of which an inorganic pigment
such as metal-free phthalocyanine pigment and
titanyl-phthalocyanine pigment is dispersed with a disperser such
as a ball mill and a dyno mill.
The charge transport layer provided on the charge generating layer
is a charge transport substance which has a capability of receiving
and transporting electrical charge which is generated by the charge
generating substance. Examples of the charge transport layer
include polycarbonate, copolymerized polycarbonate, and
polyalylate, each of which includes electron-releasing substance or
electron-accepting substance.
Examples of the electron-releasing substance include:
poly-N-vinylcarbazole and its derivative,
poly-.gamma.-carbazolylethylglutamate and its derivative,
pyrene-formaldehyde condensate and its derivative, polyvinylpyrene,
polyvinylphenanthrene, oxazole derivative, oxadiazole derivative,
imidazole derivative, 9-(p-diethylaminostyryl) anthracene,
1,1-bis(4-dibenzylaminophenyl) propane, styrylanthracene,
styrylpyrazoline, pyrazoline derivative, phenylhydrazones,
hydrazone derivative, triphenylamine compound, triphenylmetane
compound, stilbene compound, and azine compound containing
3-methyl-2-benzothiazoline ring.
Examples of the electron-accepting substance include: fluorenone
derivative, dibenzothiophene derivative, indenothiophene
derivative, phenanthrenequinone derivative, indenopyridine
derivative, thioxanthone derivative, benzo[c]cinnoline derivative,
phenazineoxide derivative, tetracyanoethylene,
tetracyanokinodimetane, bromanil, chloranil, and benzoquinone. It
is preferable that 30 to 80 weight % of the electron-accepting
substance is included in the charge transport layer.
Note that the value of the magnetic flux density indicated in this
specification is an absolute value. The developing device and the
image forming apparatus of the present embodiment are suitable for
use in an electrophotographic multifunction printer, a copying
machine, a printer, and a facsimile.
The inventors found that the generation of the development memory
at the developing sleeve can be prevented by the developing device
including: a developing roller including a rotating sleeve which
rotates while holding two-component developer composed of toner and
magnetic carrier on an outer surface having a plurality of grooves
that extend in a direction parallel to an axis of the rotation, and
a magnetic member, provided inside the rotating sleeve in such a
manner as to be unrotatable, for attracting the two-component
developer onto the outer surface of the rotating sleeve; and a
blade, provided outside the rotating sleeve with a gap from the
outer surface of the rotating sleeve for scraping off a part of the
two-component developer, the magnetic member including a magnet for
controlling a magnetic flux density in the gap to range from 70 mT
to 150 mT.
Accordingly, this developing device yields an effect of suppressing
degradation of a printed image which is caused by the generation of
the development memory.
Also, it is preferable to arrange the developing device so that the
pitch between adjacent grooves on the outer surface of the
developing sleeve ranges from 1.25 mm to 4.00 mm. With the
arrangement, it is possible to suppress the generation of the
development memory more effectively and to prevent the
deterioration in performance of the rotating sleeve to attract
(effect of scooping) two-component developer.
Further, it is preferable to arrange the developing device so that
a width of each of the grooves ranges from 0.20 mm to 0.35 mm. With
the arrangement, it is possible to prevent the deterioration in
performance of the rotating sleeve to attract (effect of scooping)
two-component developer, and to suppress unevenness in density of
an image that is finally obtained.
Further, it is preferable to arrange the developing device so that
the depth of each of the grooves ranges from 0.08 mm to 5 mm. With
the arrangement, it is possible to prevent the deterioration in
performance of the rotating sleeve to attract (effect of scooping)
two-component developer and to prevent the degradation of an image
quality caused by the accumulation of two-component developer in
the grooves in long term usage.
Further, it is preferable to arrange the developing device so that
each of the grooves has a rectangular shape in a cross section of
the rotating sleeve when the rotating sleeve is cut by a plane
perpendicular to the axis. With the arrangement, it is possible to
further prevent the generation of the development memory.
Further, it is preferable to arrange the developing device so that
the magnet is provided between the axis and the blade. With the
arrangement, the magnet is provided inside of the rotating sleeve
so as to be in a position close to the gap. Consequently, it is
possible to cause magnetism from the magnet to effectively work on
the gap, and to set the magnetic flux density of the gap to range
from 70 mT to 150 mT.
Further, it is preferable to arrange the developing device so that
the blade is made of a magnetic material. With the arrangement, the
gap is sandwiched by the magnet and the magnetic material.
Consequently, it is possible to set the magnetic flux density of
the gap to be 70 mT or more due to the synergetic effect between
the magnet and the magnetic material.
Recently, toner of small particle size, whose volume average
particle size ranges from 4 .mu.m to 7 .mu.m, is frequently used
for improving the image quality of a printed image. As more amount
of toner of small particle size is used, the generation of the
development memory is remarkably increased in the developing
sleeve. Therefore, the developing device capable of suppressing the
development memory is suitable for the image forming process using
the toner with the volume average particle size ranging from 4
.mu.m to 7 .mu.m.
Further, it is preferable to arrange the developing device so that
the volume average particle size of the magnetic carrier ranges
from 20 .mu.m to 60 .mu.m. With the arrangement, it is possible to
prevent the development memory far more effectively.
Further, it is preferable to arrange the developing device so that
the magnetic carrier is obtained by coating ferrite based core
particles with thermosetting resin. With the arrangement, it is
possible to prevent the breakage of the carrier since the resin
coating the magnetic carrier is less likely to peel off.
Further, it is preferable to arrange the developing device so that
saturated magnetization of the magnetic carrier ranges from 30
emu/g to 70 emu/g. With the arrangement, it is possible to further
prevent the generation of the development memory.
The embodiments and concrete examples of implementation discussed
in the foregoing detailed explanation serve solely to illustrate
the technical details, which should not be narrowly interpreted
within the limits of such embodiments and concrete examples, but
rather may be applied in many variations within the spirit of this
technology, provided such variations do not exceed the scope of the
patent claims set forth below.
* * * * *